1. Field of the Invention
This invention relates to a non-invasive cardiac electrode, and more specifically, to a non-invasive, radiolucent cardiac electrode which is capable of conducting energy at a level sufficient for cardiac stimulation.
2. Background Information
Prior art non-invasive electrodes adapted for defibrillating or pacing the heart typically comprise a skin-contacting pad which is laminated to a metal foil conductive backing which in turn is electrically coupled to a source of defibrillating or pacing energy. The purpose of the foil backing is to distribute the energy from the source substantially uniformly over the pad before it is delivered to the patient, thereby eliminating burns or the like which could occur if the energy were to be delivered in a more concentrated fashion. Thus, this foil backing has been viewed as a desirable, and even necessary, component of prior art non-invasive electrodes.
A problem with the prior art foil backing, however, is that it may interfere with the ability to image an aspect of the patient's circulatory system, such as the arteries of the heart, with a fluoroscope or the like, when electrodes containing the prior art backing are positioned on the patient's skin so that multiple cardiac stimulation therapies can be rapidly delivered to the patient should he or she suffer an arrhythmia during the imaging process. The interference may occur because the prior art foil backing typically creates shadows or blemishes in the images of the internal organs, all of which may impede diagnosis of the patient's condition. In the areas of cardiac catheterization, angiography, and angioplasty, for example, the practitioner must be able to see the angiography dye or the catheter to determine the extent of the presence of plaque or other irregularities in the patient's arteries. The foil backing can shadow or block the view of the patient's arteries, thereby leading to an incomplete or improper diagnosis.
Prior art radiolucent electrodes are also available which utilize a conductive rubber backing instead of metal foil. However, these rubber backing materials are suitable only for high impedance external pacing systems. They are not suitable for defibrillation as they have too high an impedance.
As a result, there has been a long-standing but heretofore unmet need for a non-invasive electrode which is radiolucent, and, at the same time, is capable of delivering multiple electrical therapies to a patient, including defibrillation therapies, in the course of an emergency cardiac resuscitation attempt. For the purposes of this disclosure, the term "radiolucent" is defined as the quality of being capable of providing a low enough attenuation of X-irradiation such that body structures such as coronary arteries imaged with intraluminal contrast material may be visualized through the electrode without significant degradation in image quality so as to make the image uninterpretable with respect to luminal irregularities, angioplasty outcome, thrombus formation, or vessel occlusion.
Although several efforts have been directed towards developing such an electrode in the past, none of these efforts have heretofore proven successful. For example, one approach, described more fully in U.S. Pat. Nos. 4,050,453; 4,257,424; 4,370,984; 4,674,511; and 4,685,467, involves forming the conductive element of a monitoring electrode by painting an electrode base with metallic paint or depositing a very thin metallic film on the base. However, this approach could not successfully be utilized in a stimulation electrode which is capable of delivering the level of energy necessary to stimulate the heart, since the thin metallic coating or film would not be capable of conducting the relatively high levels of energy required, typically 300 J or more.
A second approach, described in U.S. Pat. Nos. 4,442,315 and 4,539,995, involves forming the conductive element of the electrode by applying fine particles of an electrically conductive material, such as carbon, to an electrode base. Again, this approach could not successively utilized in a stimulation electrode, since the layer of fine particles would not be capable of conducting the relatively high levels of energy required.
A third approach, described in U.S. Pat. Nos. 4,748,993 and 4,800,887, involves forming the conductive element from a porous carbonaceous material or graphite sheet. A problem with this approach is that the resulting electrode would have too high an impedance when used for defibrillation in that the high levels of energy would cause the electrode to heat up too much.
Accordingly, it is an object of the present invention to provide a radiolucent non-invasive cardiac electrode which is capable of delivering multiple electrical stimulation therapies, including defibrillation, to a patient in the course of an emergency cardiac resuscitation attempt.